1. Field of the Invention
This invention relates to optical networks.
2. Description of the Related Art
FIG. 1A of the accompanying drawings shows in block diagram form the basic components of a passive optical network (PON). A multiwavelength optical source 3, located in a central office 1, transmits light signals consisting of multiple discrete wavelengths λ1 . . . λN down an optical fibre 10 to a wavelength division multiplexer (WDM) 7, located in a remote node 5, which then distributes the signals to a set of optical network units (ONUs) 9, via separate fibres 11. The network is described as passive since the optical routing components (such as the WDM 7) cannot actively be controlled or tuned during their operational use.
The wavelength division multiplexer 7 may be one of a variety of types. An example of a simple multiplexer is a power-splitting star coupler which simply splits incoming light into all ports equally; it is the trivial case of wavelength division multiplexing, because no selection is made on the basis of wavelength, and consequently all-wavelengths λ1 . . . λN are distributed to all ONUs 9, as illustrated in FIG. 1B of the accompanying drawings. This arrangement is sometimes referred to as “broadcast-and-select”, since each signal is broadcast to multiple ONUs 9, and each ONU 9 then selects only those signals intended for it.
Instead of such a power-splitting star coupler, a wavelength routing element, for example an arrayed waveguide grating (AWG), could be used. An AWG splits incoming light into spectral constituents, launching them onto separate output fibres. In this way, with an appropriately-designed AWG, incoming light consisting of wavelengths λ1 . . . λN could be multiplexed into N separate branches each consisting of light of only one of those wavelengths, as illustrated in FIG. 1C of the accompanying drawings. In this way, each ONU 9 would only receive signals intended for that ONU, and each output branch would receive all the incoming power for its designated wavelength, unlike the star coupler where there is a splitting of power. Note that the architecture in FIG. 1C shows the case where there are the same number of ONUs 9 as there are wavelengths emitted from the source 3, but this is not necessary; for example an ONU 9 could receive more than one of the routed wavelengths.
FIG. 2 of the accompanying drawings shows an example of a recently-proposed two-stage wavelength-routed PON architecture having a multiwavelength optical source 3 at the optical line termination (OLT) emitting discrete wavelengths λ11 . . . λMN down fibre 10. In the illustrated architecture there is one coarse AWG 4 located in an exchange 2, and M remote nodes 5, each having a fine AWG 7. Each fine AWG 7 feeds N ONUs 9, so that there are a total of M×N ONUs 9.
The coarse AWG 4 is designed to direct multiple wavelengths down each branch 6, and these wavelengths are then separated by the fine AWGs 7 and directed individually to each ONU 9 via the branches 11. This is achieved by ensuring that the free spectral range of the coarse AWG 4 is equal to the spacing of N channels received by the branches 11.
For example, using the illustrated architecture of FIG. 2, the coarse AWG 4 receives at its input all wavelengths λ11 . . . λMN emitted from the source 3. It directs wavelengths λ11 . . . λ1N down the first branch 6 to the first remote node 5. The AWG 7 within the first remote node then directs each of the N wavelengths λ11 . . . λ1N at its input individually to the N respective ONUs 9.
In this architecture, like that of FIG. 1C, each ONU receives only the wavelength assigned to it, and for each wavelength there is no splitting of power at the routing components 4, 7.
The multiwavelength optical source 3 may be a single tunable laser located in the optical line termination (OLT) of fibre 10, constantly retuning and transmitting a different wavelength in different time slots. This scheme uses WDM principally to improve the privacy in the network; there is no increase in capacity over a single wavelength system as only one wavelength is transmitted every time slot. The downstream protocol is effectively the same as a time division multiplexed (TDM) single wavelength system.
The above-described architectures of FIGS. 1C and 2 are fixed wavelength systems, since a wavelength is permanently assigned to each branch of the PON, effectively creating a number of independent single wavelength networks within the same PON. This type of scheme is simple to implement but does not allow the redistribution of bandwidth in response to fluctuations in demand. For example, if the n'th ONU 9, permanently assigned wavelength λn, is idle for a long period of time, then that wavelength λn is being wasted since it cannot be re-allocated to another ONU 9.
Dynamic assignment schemes seek to allow more flexible use of bandwidth by introducing tunability into the network. The most obvious way to provide downstream wavelength re-allocation is to have tunable filters in the ONUs 9, in a broadcast-and-select architecture such as that of FIG. 1B where each ONU 9 receives more than one wavelength. The ONUs 9 of the PON would tune to the wavelength assigned to it in response to a signal from the central office 1.
There is, however, a major drawback with this approach to dynamic assignment of wavelengths, which is that the information about current bandwidth requirements is held at the central office 1, and is separated from the location of the tunable components in the ONUs 9. Therefore when a retuning is required, a signal needs to be sent from central office 1 to the appropriate ONU 9, and an acknowledgement returned, before data destined to that ONU 9 can be transmitted on the new wavelength. As retuning is normally done in response to the overloading of a wavelength channel, this lag causes a build up of traffic and consequent increase in delay on that channel.
The present applicant has considered employing more than one tunable laser at central office 1 (the “head end”) in a fixed wavelength PON so as to achieve a certain degree of dynamic bandwidth assignment without the use of tunable filters in the ONUs 9. With multiple tunable lasers, transmission to the ONUs 9 could be shared between the lasers. Each laser could be assigned its own set of ONUs to which to transmit, and consequently when the load on a particular laser is increased, for example due to an increase in demand from a particular ONU 9, responsibility for transmission to that ONU could be transferred to another less loaded laser. The effect would be effectively to transfer the tunability in the network from the ONUs to the head end.
There would be a number of advantages in doing this. Firstly, the tuning would be done with tunable transmitters rather than filters, the former currently having a faster tuning speed. Secondly, all the protocol functions would be controlled at the head end. Consequently, having the tuning there would mean that there is no delay between the tuning becoming necessary and it being implemented. This could stop traffic build up on an overloaded transmitter as discussed above. Thirdly, the system would be more robust; if the tuning is at the ONU 9 then either an acknowledgement of successful retuning is required, resulting in further delay, or there is the risk of an error in retuning resulting in the loss of cells transmitted to the ONU 9 on the new wavelength. Fourthly, the more expensive, tunable components would be placed at the head end, where only a few are required, rather than providing expensive tunable systems at each ONU; this would lead to a cost reduction.
There are still certain drawbacks, however, to such a fixed-filter, tunable-laser approach. Firstly, cells could be addressed to more than one ONU 9. This means that bandwidth would be wasted when the network transmits broadcast or multicast traffic, because the cell needs to be replicated and retransmitted on the wavelength of each destination ONU 9. In contrast, a system with tunable filters at the ONU could be configured so that all the ONUs 9 in a multicast group can be tuned to the same channel. Secondly, constant retuning of the lasers at the head end would be required. Consequently, if the tuning time is non-negligible, then a loss of bandwidth would result.
It is therefore desirable to provide a multiwavelength, broadcast-and-select optical network which combines head end tuning with efficient transmission of broadcast and multicast traffic.